ET and water usage is not a function of root depth. You might say it is, as it regards the ET value for trees versus shrubs versus grass, but each plant type has its own Crop Coefficient, which takes that part of it into account. Soil type doesn’t really affect it, although mulch or ground covering does.
Basically, ET is calculated from the FRET ( Forecast Reference crop EvapoTranspiration), which is the estimated depth of water that would evaporate and transpire from a reference crop under the forecast weather conditions on a daily basis. While it may be an actual Crop, I’ve heard it defined as the water that would evaporate from a pan of water sitting on the ground. Temperature, humidity, wind, sun, etc. affect that value.
Then the ET is calculated from the FRET value by multiplying it by the Crop Coefficient. A sun/shade coefficient is also considered, maybe something more.
Anyhow, for a given type of crop, the root depth does not play into how quickly water is used (the ET factor).